1. Field of the Invention
The present invention relates to a controlled magnetic bearing apparatus for supporting a rotatable member in a levitated state by a magnetic force, and more particularly to a magnetic bearing apparatus which can effectively maintain stiffness of a magnetic bearing. The present invention also relates to a fluid machine having such a magnetic bearing apparatus and a motor for rotating a rotatable member.
2. Description of the Related Art
Magnetic bearing devices have heretofore been widely employed in a fluid machine having a rotatable member to be rotated at high speeds because magnetic bearing devices allow the rotatable member to be supported in a non-contact manner by a magnetic force. Such a magnetic bearing device has the following advantages. It is possible to reduce a rotational resistance of the rotatable member supported by bearings. No particles are produced by abrasion of bearings. Maintenance of bearings by abrasion is not required. There is no contamination caused by a lubricant for bearings.
For example, a gas laser apparatus 500 shown in FIG. 1 has a laser container 501 containing a laser gas therein and a circulating fan 503 disposed within the laser container 501. The laser gas includes a halogen gas such as a fluorine gas. The circulating fan 503 has a rotatable shaft 504, which projects outward from both ends of the laser container 501. The rotatable shaft 504 is supported by magnetic bearing devices and rotated by a motor 505. Specifically, the rotatable shaft 504 is rotatably supported in a non-contact manner away from the laser container 501 by radial magnetic bearing devices 506 and 507 provided at both ends of the laser container 501 and an axial magnetic bearing device 508 provided at one end of the laser container 501.
Each of the magnetic bearing devices 506, 507, and 508 generally has the same structure, which includes a magnetic target provided on the rotatable shaft 504 and an electromagnet provided on the laser container 501 at a position facing the magnetic target so as to levitate and support the rotatable shaft 504 in a non-contact manner away from the laser container 501 by a magnetic force of the electromagnet. Thus, only the radial magnetic bearing device 507 will be described below.
The radial magnetic bearing apparatus 507 has a displacement sensor 507a, an electromagnet 507b, a displacement sensor target 507c, and an electromagnet target 507d made of a magnetic material. The displacement sensor 507a and the electromagnet 507b are provided on the laser container 501, and the displacement sensor target 507c and the electromagnet target 507d are provided on the rotatable shaft 504 of the circulating fan 503. Thus, the rotatable shaft 504 is supported in a non-contact manner away from the laser container 501 by a magnetic attractive force of the electromagnet 507b. 
The radial magnetic bearing device 507 includes a control device. The control device includes a displacement detector for detecting a relative position of the rotatable shaft 504 based on a signal from the displacement sensor 507a, which detects the displacement sensor target 507c, a phase compensator for calculating and outputting a bearing control signal according to a deviation between the detected position of the rotatable shaft 504 and a reference position so as to stably position the rotatable shaft 504 at a predetermined location, and a driver for amplifying and supplying the bearing control signal as an exciting current to the electromagnet 507b. The control device allows the rotatable shaft 504 of the circulating fan 503 to be positioned at a predetermined location and to thus be rotated stably by the motor 505.
Various efforts have been made in controlling operations of fluid machines having such magnetic bearing devices in order to stably support a rotatable member rotated at a high speed in a non-contact manner.
However, in a fluid machine having conventional magnetic bearing devices, when the rotatable shaft 504 is rotated by the motor 505 in the gas laser apparatus 500, an unbalanced radial magnetic pull is produced so as to lower an open-loop gain of the magnetic bearings. Accordingly, the stiffness of the magnetic bearings is adversely lowered.
FIG. 2 is a graph showing characteristics of the magnetic bearing stiffness of a conventional magnetic bearing device. In FIG. 2, the stiffness of the radial magnetic bearing 507 with a driving force of the motor 505 is compared to that without a driving force of the motor 505. As shown in FIG. 2, the stiffness of the radial magnetic bearing 507 is lowered by an unbalanced radial magnetic pull of the motor 505 produced when the motor 505 is driven. Particularly, the stiffness of the radial magnetic bearing 507 is lowered near a critical speed in a rigid mode of the rotatable shaft 504 (see X in FIG. 2). If the stiffness of a magnetic bearing is lowered near a specific frequency, then the rotatable shaft 504 excessively whirls within a range including the specific frequency. Thus, the rotatable shaft 504 cannot be rotated stably. This tendency becomes more significant as an output of the motor 505 is increased.
The above problems cannot be solved by the following conventional methods of controlling a magnetic bearing. For example, a band-pass filter is used to prevent unbalance of a rotatable member when the rotatable member is rotated at a high speed and to enhance the stiffness of a magnetic bearing when the rotatable member is rotated at a low speed. In this case, although a gain for levitation control may be adjusted near a rotation frequency of the rotatable member so as to be lower than a gain at other frequencies, it cannot be adjusted so as to be higher than a gain at other frequencies. Accordingly, this method is not effective in passing a critical speed in a rigid mode within a low-frequency range, in which the rotatable member is not rotated about the center of inertia. Further, such a control requires complicated circuits.
Further, there has been developed a method for improving the bearing stiffness at a slip frequency of an induction motor. However, such a method is not effective in improving the bearing stiffness near a critical speed of a rotatable member. There has also been developed a method of detecting a current supplied to a motor and enhancing the bearing stiffness when the detected current is larger than a reference value. However, such a method cannot control a gain for levitation control near a specific frequency at which the bearing stiffness is lowered.
The following methods have been proposed to solve the above drawbacks. An open-loop gain to be lowered due to an unbalanced radial magnetic pull caused by a motor is added to a proportional gain for levitation control in advance. Alternatively, a proportional gain for levitation control is increased according to load conditions of a motor. However, these methods lose the control stability at higher-order frequencies in a flexible mode. Thus, it is difficult to put these methods into practice.
Further, when the rotatable shaft 504 is rotated, a critical speed diverges into a critical speed at a lower frequency and a critical speed at a higher frequency due to gyroscopic effects. As the rotation frequency of the rotatable shaft 504 is higher, a difference between a backward whirl frequency on a lower side and a forward whirl frequency on a higher side becomes larger. When the rotatable shaft 504 is not rotated, the magnetic bearing can be controlled stably. However, when the rotatable shaft 504 is rotated, critical speeds vary according to the rotation frequency of the rotatable shaft 504. Particularly, the control of the bearing is likely to be unstable at a forward whirl frequency and a backward whirl frequency, which correspond to critical speeds in a flexible mode of the rotatable shaft 504. If a proportional gain is lowered in order to maintain the stability of the magnetic bearing, then the bearing stiffness is lowered in a low-frequency range to thereby cause an excessive whirl of the rotatable shaft 504. Thus, the control stability of the magnetic bearing cannot be achieved at both of lower rotation frequencies and higher rotation frequencies.
Further, sufficient bearing stiffness cannot be maintained near a critical speed in a flexible mode. Accordingly, in order to rotate the rotatable shaft 504 at frequencies higher than the critical speed in the flexible mode, accurate balancing, which has not been practical, is required. Although there has been proposed to lower a control gain at the critical speed in the flexible mode so as to stably support a rotatable member, it is difficult to rotate the rotatable member at frequencies higher than the critical speed in the flexible mode.